The high energy efficiency is one of the most critical requirements for wireless sensors since the systems are usually located in the energy-limited environments and their primary energy sources are limited to only several options: batteries, wireless power transfer methods, or a combination of them. Their lifetime is mostly determined by the battery lifetime, and those systems must be also replaced after a certain time period with the expensive procedures.
The low power techniques to extend the lifetime of those systems mostly depend on the aggressive power supply scaling such as dynamic voltage scaling (DVS) techniques. While the scaled supply makes it possible for the systems to accomplish economic energy usage, the squeezed voltage headroom of the circuits incurs poor power supply rejection ration (PSRR) of the systems. To remedy this problem, linear regulators (linear DC-to-DC converter) are widely employed in the conventional power management of the sensor systems for the stable regulation of power supplies. However, in the battery-powered system the power management with linear regulators fails to secure high power conversion efficiency because they dissipate away the large drop-out voltage resulted from the battery voltage (3˜5V) and the required supply (0.5˜1.5V). Therefore, other type of power converters should be investigated to exploit the recent low power techniques, and meanwhile guaranteeing high power supply integrity.
In broad sense, the power converter can be divided into two groups: linear power converter (regulator) and switching power converter. The latter is further classified into switched capacitor (SC) converters and inductive power converters. Generally speaking, the switched capacitor power converters can be fully integrated with other circuits on a single chip and show relatively high power conversion efficiency (60-90%). However, they only provide a discrete voltage conversion ratio (VCR, ratio of the output to input voltage) and require complex compensation techniques to generate wide range of VCRs. In the battery operating system which usually requires wide range of VCR, that disadvantage is difficult to overcome. On the other hand, the inductive power converters can provide all possible VCRs in a given topology (for example, buck, boost, fly-back and so on) and exhibit high power conversion efficiency in general. Even though the inductive power converters need several external components (typically 2 external components for basic topologies), it is tolerable for the sensor considering the typical form factor of the systems. Therefore the inductive power converter can be a good candidate instead of linear regulators for the battery-powered sensor systems.
In terms of the operating frequency of the inductive power converters, the controls mechanism of the converters can be further classified into the two different schemes: pulse frequency modulation (PFM) and pulse width modulation (PWM). The PFM power converters can provide high power conversion efficiency (˜90%) for wide range of the loads due to their adjustable switching frequency with loading conditions. However, the PFM control has serious shortcomings when combined with power noise-sensitive loads. For instance, the output ripples are larger by the lower switching frequency, which magnifies the impact of switching noise on the load. Moreover, the frequency of the output spurs is a strong function of load current, and therefore their location will vary from a specific operation to another. Thus, predicting their precise location in order to mitigate their impact on the load side becomes extremely difficult. On the other hand, the PWM power converters can provide the stable output with relatively fast transient while exhibiting high power conversion efficiency over medium to high loads. In addition, their output spurs are predictable due to their fixed switching nature, therefore the PWM power converter are able to provide clean supplies once including proper filtering. However, their power conversion efficiency usually suffers at light load condition, in particular less than a few mW that most of the sensor systems consume. In operations, where a few of the signal processing blocks of the systems are the only active parts in the system, low power conversion efficiency is inevitable with the PWM power converters.
For instance, when the power and energy consumption of an implantable biomedical sensors are considered this phenomenon is clearly observed as shown in FIG. 1A where the percentages of power consumptions of the common functional blocks in the implantable biomedical systems are described. Since the analog front-end (AFE) always operates while the other blocks such as data communications and electrical/optical stimulations are active based on the systems request, the operation of the power conversion (dc-to-dc conversion) is mostly dedicated to the low power regions. Therefore high energy efficiency is not able to be achieved if the conventional PWM power converter is adopted for the systems. Moreover, considering the battery life time, the energy consumption should be considered instead of the peak power consumption. Based on the estimated values from FIG. 1A, the daily energy consumption can calculated as shown in FIG. 1B. Even though the AFE only consumes a small fraction of the total power consumption, around 0.97%, it takes about 87% of the total daily energy consumption. Hence, if the power conversion efficiency on low power region is low, the battery life time becomes shorter. In conclusion, it appears that high energy efficiency cannot be achieved if the conventional PWM power converters are employed.
This section provides background information related to the present disclosure which is not necessarily prior art.